This invention relates to a catalytic process for hydrotreating heavy hydrocarbon streams containing asphaltenic material, metals and sulfur compounds. More particularly, this invention relates to hydroprocessing using catalytic treatment with catalyst having a bimodal pore structure and improved effectiveness in the desulfurization and demetallation of metal-containing hydrocarbon streams.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, and sulfur.
It is widely known that various organometallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life. The various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst will become deactivated rapidly and will be subject to premature replacement.
Hensley et al in co-pending U.S. patent application Ser. No. 928,141 filed July 26, 1978, which is incorporated herein by reference, disclose a multiple-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and a substantial amount of metals. The first stage of this process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a large-pore, high surface area inorganic oxide support; the second stage of the process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of hydrogenation metal selected from Group VIB deposed on a smaller pore, catalytically active support comprising alumina, said second stage catalyst having a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, an average pore diameter within the range of about 90 A to about 160 A, and the catalyst has a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm. Hensley et al disclose that as little as 2.2 wt.% cobalt oxide caused more rapid deactivation of their second-stage catalyst for sulfur removal. The catalyst employed in the process of this invention comprises a bimodal pore distribution, that is, the catalyst contains a large fraction of its pore volume in micropores having diameters within the range of about 50 A to about 200 A and substantial macropore volume in pores having diameters of 600 A or greater, but in addition, the catalyst contains a minimum of its micropore volume in pores having diameters within the range of about 200 A to about 600 A; this catalyst enables the process of this invention to provide improved hydrodemetallation and hydrodesulfurization.
Examples of multiple-stage catalytic processes for hydrotreatment of heavy hydrocarbon streams containing metals are disclosed in U.S. Pat. Nos. 3,180,820 (Gleim et al., 1965); 3,730,879 (Christman, 1973); 3,977,961 (Hamner, 1976); 3,985,684 (Arey et al., 1977); 4,016,067 (Fischer, 1977); 4,054,508 (Milstein, 1977); 4,051,021 (Hamner, 1977) and 4,073,718 (Hamner, 1978).
The catalysts disclosed in these references contain hydrogenating component comprising one or more metals from Group VIB and/or Group VIII on high surface area support such as alumina, and such combinations of metals as cobalt and molybdenum, nickel and molybdenum, nickel and tungsten, and cobalt, nickel and molybdenum have been found useful. Generally, cobalt and molybdenum have been preferred metals in the catalysts disclosed for hydrotreatment of heavy hydrocarbon streams, both in first-stage catalytic treatment to primarily remove the bulk of the metal contaminants, and in second-stage catalytic treatment primarily for desulfurization. None of the references disclose actual examples of processes employing catalyst containing only Group VIB metal; some of the references, for example the aforementioned U.S. Pat. No. 3,977,961, disclose catalyst micropore structures considered to have beneficial catalyst performance; however, none of the references disclose or suggest the improved performance and bimodal pore structure of the catalyst employed in the process of this invention.
Hopkins et al. in U.S. Pat. No. 4,119,531, which is incorporated herein by reference, disclose a process for hydrodemetallation of hydrocarbon streams containing asphaltenes and a substantial amount of metals, which comprises contacting the hydrocarbon stream with a catalyst consisting essentially of a small amount of a single hydrogenation metal from Group VIB or Group VIII, deposed on a large pore alumina; suitable examples of the hydrogenation metal are nickel or molybdenum. The catalyst is characterized by a surface area of at least 120 m.sup.2 /gm; a pore volume of at least 0.7 cc/g and an average pore diameter of at least 125 A units. Hopkins et al. suggest that while hydrodemetallation of heavy hydrocarbon streams is improved by employing catayst consisting essentially of a single Group VIB or Group VIII hydrogenation metal, the substantially demetallated effluent will not normally be sufficiently desulfurized for further refining processes.
U.S. Pat. No. 4,016,067 (Fischer, 1977) discloses a process for removing metal and sulfur contaminants from residual oil fractions by contacting the oil in a dual catalytic bed system wherein the first catalyst comprises a Group VIB metal and an iron group metal oxide, such as a mixture of cobalt and molybdenum oxides, composited with an alumina support that contains delta or theta phase alumina, the catalyst having at least 60% of its pore volume in pores of 100 A to 200 A diameter, at least about 5% of its pore volume in pores having a diameter greater than 500 A, and a surface area up to about 110 m.sup.2 /gm. The oil is then contacted with a second catalyst of the high-surface area, cobalt-molybdenum on alumina type with a major fraction of its pores in the 30 to 100 A diameter range. Fischer discloses that the preferred first catalyst and its effectiveness in his process "is believed to be associated with a relatively low surface area of up to about 110 m.sup.2 /gm and the presence of a demonstrable content of a high temperature phase alumina, i.e., a delta and/or theta alumina." In contrast, the process of this invention employs bimodal catalyst containing only Group VIB metal and in further contrast such catalyst has a surface area of at least about 140 m.sup.2 /gm and a micropore structure containing less than about 15% of its micropore volume in pores having diameter 200-600 A, as well as significant macropore content.
U.S. Pat. No. 3,898,155 (Wilson, 1975) discloses a process for the simultaneous demetallization and desulfurization of heavy oils (containing at least 50 ppm metals) under hydrogenation conditions by employing the catalyst composition comprising a Group VI metal and at least one Group VIII metal composite with a refractory oxide, said catalyst composition having from 10% to 40% of the total pore volume in macropores (pores having a pore diameter greater than 600 A as determined by mercury porosimetry) and from 60% to 90% of the total pore volume in micropores (pores having a diameter in the range 0-600 A as determined by the nitrogen adsorption method) and at least 80% of the micropore volume being in pores having a diameter of at least 100 A. In contrast, the process of this invention employs bimodal catalyst containing only Group VIB metal and in further contrast such catalyst contains less than about 75% of its micropore volume in pores having diameters between 100 and 600 A and less than about 15% of its micropore volume in pores having diameter 200-600 A.
The general object of this invention is to provide an improved process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing metals and asphaltenes. Another object of this invention is to provide improved catalyst for use in such processes.
We have found that the objects of this invention can be obtained in a process for hydrodemetallation and hydrodesulfurization of hydrocarbon feedstock containing asphaltenes and metals by contacting said feedstock with hydrogen and a bimodal catalyst consisting essentially of at least one active original hydrogenation metal selected from Group VIB deposited on a support comprising alumina wherein said catalyst has a surface area within the range of about 140 to about 300 m.sup.2 /gm, a total pore volume based upon measurement by mercury penetration within the range of about 0.4 cc/gm to about 1.0 cc/gm, and comprising about 60% to about 95% of its micropore volume in micropores having diameters within the range of about 50 A to about 200 A, 0% to about 15% of its micropore volume in pores having diameters within the range of about 200 A to about 600 A and about/3% to about 30% of said total pore volume in macropores having diameters of 600 A or greater.
The term "active original hydrogenation metal" is used herein to refer to only the hydrogenation metal that is incorporated into the catalyst during its preparation and does not include any metal that is deposited upon the catalyst during the use of the catalyst in any process. Molybdenum, which is generally superior to chromium and tungsten in demetallation and desulfurization activity, is a preferred Group VIB metal component in the bimodal catalyst.
The bimodal catalyst employed in the process of this invention comprises a pore structure having a bimodal character by which is meant the pore structure comprises a concentration of the pore size distribution in a particularly important range of specified micropores and a significant fraction of macropores, with a minimum of pores having a size intermediate between these concentrations. The terms "micropores" and "micropore volume" are used to refer to that portion of the entire catalyst pore volume contained in pores having a diameter in the range 0 to about 600 Angstrom (A) units as determined by measurement by nitrogen desorption technique derived from the method described by E. V. Ballou, O. K. Dollen in "Analytical Chemistry," Vol. 32, page 532, 1960. The terms "macropores" and "macropore volume" are used to refer to that portion of the entire catalyst pore volume contained in pores having a diameter of 600 A or greater, as determined by measurement by mercury penetration (porosimetry) technique based upon the procedure described by Winslow and Shapiro in ASTM Bulletin; February, 1959.
The bimodal catalyst employed in the process of this invention can be prepared with a support comprising alumina, preferably gamma-alumina, having BET surface area within the range of about 140-300 m.sup.2 /gm and having exemplary micropore volume distribution ranges, determined by nitrogen desorption technique, as follows:
______________________________________ Pore diameters, A % of Micropore Volume ______________________________________ 50-200 60-95 200-600 &lt;15 ______________________________________
The alumina should have pore volume distribution ranges determined by mercury penetration technique as follows:
______________________________________ Pore diameters, A % of Total Pore Volume ______________________________________ 0-200 60-95 200-600 0-15 600-10,000 3-30 ______________________________________
The support for the bimodal catalyst of this invention is preferably alumina; however, the support can comprise silica, phosphate, or other porous refractory inorganic oxide, preferably in amount less than about 5 wt.% of the support. The gamma-alumina support can be subsequently treated by the typical commercial method of impregnation with a solution or solutions, usually aqueous, containing the heat-decomposable salts of Group VIB metal. A preferred Group VIB metal is molybdenum which is generally superior to chromium and tungsten in desulfurization activity; combinations of the Group VIB metals can also be employed. The hydrogenation metal can be present in the catalyst in an amount within the range of about 3 wt.% to about 30 wt.% or greater, calculated as the oxide of the respective metal and based upon the total catalyst weight. Preferably, the metal is present in an amount of about 5 wt.% to about 15 wt.%.
Suitably, the finished bimodal catalyst employed in the process of this invention can have a BET surface area within the range of about 140 to about 300 m.sup.2 /gm, a total pore volume based upon measurement by mercury penetration within the range of about 0.4 cc/gm to about 1.0 cc/gm, and comprising about 60% to about 95% of its micropore volume in micropores having diameters within the range of about 50 A to about 200 A, 0% to about 15% of its micropore volume in pores having diameters within the range of about 200 A to about 600 A and about 3% to about 30% of its total pore volume as determined by mercury penetration in macropores having diameters of 600 A or greater.
Preferably the bimodal catalyst has a surface area within the range of 150 to 250 m.sup.2 /g, said total pore volume within the range of about 0.5 cc/g to about 1.0 cc/g and comprising about 70% to about 90% of its micropore volume in micropores having diameter within the range of about 50 to about 200 A, 0 to about 10% of its micropore volume in micropores having diameters within the range of about 200 to about 600 A, and about 3.0% to about 15% of said total pore volume in macropores having diameters of 600 A or greater.
Most preferably, the bimodal catalyst has a surface area within the range of about 150 to 250 m.sup.2 /g, said total pore volume within the range of about 0.5 cc/g to about 1.0 cc/g, and comprising about 50% to about 90% of its micropore volume in micropores having a diameter within the range of about 80 A to about 150 A, 0% to about 10% of its micropore volume in micropores having diameters within the range of about 200 to about 600 A, and about 3% to about 20% of said total pore volume in macropores having diameters of 600 A or greater.
Broadly, the present invention is directed to a process for the hydrotreating of heavy hydrocarbon feedstocks. Such feedstocks will contain asphaltenes, metals, nitrogen compounds, and sulfur compounds. It is to be understood that the feedstocks that are to be treated by the process of the present invention will contain from a small amount of metals such as nickel and vanadium, e.g., about 40 ppm, up to more than 1,000 ppm of the combined total amount of nickel and vanadium and up to about 25 wt.% asphaltenes.
When the feedstock to be processed contains a very large amount of metals, generally 100 ppm and typically 150 ppm or more of metals such as nickel and vanadium, the feedstock can be treated in a sequential, two-stage method wherein a demetallation catalyst is employed in a first-stage to provide a partially demetallated effluent which is contacted in a second-stage with the bimodal catalyst employed in this invention which then substantially removes the bulk of the sulfur and remaining metals. Generally if the feedstock to be processed contains less than about 200 ppm and preferably less than about 150 ppm of metals such as nickel and vanadium, the bimodal catalyst alone can be employed without prior partial demetallation of the feedstock.
In the reaction zones, catalysts may be employed in the form of a fixed-bed or an ebullated bed of particles. In the case of a fixed-bed, the particulate material catalyst should have a particle size of at least 1/32 to about 1/8 inch (0.08-0.32 cm) effective diameter.